The present invention relates generally to the extraction of energy values from oil shale and, more particularly, to a method for sequentially retorting and combusting oil shale containing calcium carbonate to effect efficient extraction of heat values from the oil shale.
With the increasing energy demands of the world and the decreasing reserves of conventionally recovered petroleum, alternative sources of energy are being extensively investigated as substitutes for energy extracted from petroleum. For example, oil shale contains a considerable concentration of volatile hydrocarbons which are recoverable from the shale by subjecting the shale to a sufficient temperature to drive off the volatile hydrocarbons contained therein. These volatile hydrocarbons may then be used in a combustion system as the primary fuel source. A significant problem which detracts from the efficiency of using oil shale for supplying volatile hydrocarbons for combustion purposes is that most oil shales contain a considerable concentration of calcium carbonate (limestone). Common of such oil shales are those found in vast quantities in areas such as Colorado, Israel, and Morocco. When oil shales containing considerable concentrations of calcium carbonate are burned in a direct combustion process, the calcium carbonate tends to undergo calcination which is an extensive heat-consuming reaction during which calcium carbonate is converted to lime. For each pound of calcium carbonate that is calcined as much as 600 to 700 BTU's of the available heat energy is consumed. This extensive heat consumption represents greater than about 30% of the total heat energy available in each pound of the Israeli oil shale and as much as about 24% of the heat energy available in each pound of the Colorado oil shale. Inasmuch as the limestone content in most oil shales such as mentioned above amounts to more than about 50% of the total oil shale composition, it becomes of increasing importance from the efficiency standpoint to effect the retorting or the separation of the hydrocarbons from the oil shale without simultaneously undergoing the calcination reaction to avoid the extensive loss of available heat energy suffered during concurrent calcination and combustion of the oil shale.